The present invention relates to a two-dimensional image detector that is suitably used for detecting a two-dimensional image of light (radiation rays) such as X-rays, visible light, or infrared rays, and a fabrication method of the same.
Conventionally well-known as a detector for detecting a two-dimensional image of radiation rays is a two-dimensional image detector in which a plurality of semiconductor sensors that detect X-rays projected thereto and generate electric charges (electron-hole) are disposed on a two-dimensional plane, each semiconductor sensor being equipped with an electric switch. The two-dimensional image detector is arranged so as to detect a two-dimensional image by successively turning on the electric switches for each row to read electric charges generated by the semiconductor sensors for each column.
A principle and specific structure of the foregoing two-dimensional image detector is taught by, for instance, by the following documents: D. L. Lee et al., xe2x80x9cA New Digital Detector for Projection Radiographyxe2x80x9d, SPIE, 2432, pp.237-249, 1995xe2x80x9d; L. S. Jeromin et al., xe2x80x9cApplication of a-Si Active Matrix Technology in a X-ray Detector Panelxe2x80x9d, SID 97 DIGEST, pp.91-94, 1997; and the Japanese Publication for Laid-Open Patent Publication No. 342098/1994 (Tokukaihei 6-342098 [Date of Publication: Dec. 13, 1994]). The foregoing two-dimensional image detector is formed as follows: a photoconductive layer that absorbs X-rays to generate electric charges is formed on an active matrix substrate having a plurality of electrode wires arranged in XY matrix, and switching elements and pixel electrodes which are provided at intersections of the foregoing electrode wires, and further, bias electrodes are formed on the foregoing photoconductive layer.
Further, for example, the foregoing document recites using a-Se (amorphous selenium) that has good sensitivity to X-rays and can easily be formed on a large-area substrate as a material for forming the photoconductive layer. a-Se exhibits a high X-ray absorption rate and a high conversion rate of X-rays to electric charges, and also, it is directly formed on an active-matrix substrate at a relatively low temperature by vapor deposition.
To improve the S/N ratio of the two-dimensional image detector, the quantity of the electric charges generated by X-ray absorption should be increased. However, to increase the quantity of electric charges, it is necessary to form the photoconductive layer to a thickness of about 500 xcexcm to 1500 xcexcm. Formation of such a thick photoconductive layer by vapor deposition, however, takes long time, and further, management of the process is complex. As a result, productivity suffers greatly, and the manufacturing costs of the two-dimensional image detector is increased. Moreover, in the method in which a photoconductive layer is directly provided on the active matrix substrate, the active matrix substrate is heated when forming the photoconductive layer. Therefore, the heat resistance (heat resistance temperature) of the active matrix substrate has to be taken into consideration. Therefore, in the foregoing method, it is difficult to use a material (for instance, CdTe, CdZnTe) which needs to be deposited at high temperatures as a material of the photoconductive layer.
A method intended to solve the foregoing problems is disclosed, for example, in the Japanese Publication for Laid-Open Patent Application No. 211832/1999 (Tokukaihei 11-211832 [Date of Publication: Aug. 6, 1999]) which teaches forming a photoconductive layer by a coating process, rather than vapor deposition. More specifically, in the foregoing method, a particle-dispersed material that is formed by dispersing particulate photoconductors in a binder such as insulating resin is coated to a thickness of about 700 xcexcm to 3000 xcexcm over the active matrix substrate, to form a photoconductive layer. This method allows the use of various materials (photoconductive materials) that exhibit high X-ray absorption rate and high X-ray-charge conversion rate, apart from the aforementioned a-Se, as the material of the photoconductors. Therefore, the photoconductive layer can be formed by combining such a material and the binder such as resin. Incidentally, since the foregoing method allows the photoconductive layer to be formed in a short time, a high productivity may be achieved while reducing the manufacturing cost of the two-dimensional image detector.
However, forming the photoconductive layer by the method as disclosed in the Japanese Publication for Laid-Open Patent Application No. 211832/1999 (Tokukaihei 11-211832) is bound to various problems which are commonly associated with the coating process. That is, in the case of forming the photoconductive layer on a large-area substrate by the spin-coating method, the thickness of the layer which can be evenly formed by a single spin-coating cannot be increased by more than around several microns. Further, in the case of forming the photoconductive layer on the substrate by a screen printing method, the thickness of the layer which can be evenly formed by a single screen printing cannot be increased by more than several ten microns. Thus, in order to form a photoconductive layer having a thickness of around 700 xcexcm to 3000 xcexcm by these coating methods, the coating process needs to be repeated at least ten to several ten times. As a result, the coating process takes time and the management of the process becomes complex, which results in low productivity and increased manufacturing cost of the two-dimensional image detector. Further, since the resulting photoconductive layer has a laminated structure, the thickness and composition of the photoconductive layer with respect to the entire substrate may become non-uniform. Similarly, it is also very difficult to evenly form the photoconductive layer of the foregoing thickness over the entire substrate by the method such as a roll-coater method, offset printing method, or spraying method.
That is, the conventional methods employing the coating process have a difficulty in productively (efficiently) and inexpensively manufacturing a two-dimensional image detector having superior uniformity in thickness and composition of the photoconductive layer with respect to the entire substrate.
The present invention was made to solve the foregoing problems and it is an object of the present invention to provide a two-dimensional image detector having superior uniformity in thickness and composition of a photoconductive layer with respect to the entire substrate, and a fabrication method of productively (efficiently) and inexpensively manufacturing such a two-dimensional image detector.
A first object of the present invention is to provide a two-dimensional image detector having superior uniformity in thickness and composition of a photoconductive layer with respect to the entire substrate.
In order to achieve the foregoing object, a two-dimensional image detector in accordance with the present invention includes at least an active matrix substrate having a plurality of pixel electrodes, and a photoconductive layer stacked on the pixel electrodes, wherein the photoconductive layer is transferred onto the active matrix substrate after being formed on a transfer substrate.
With this arrangement, since the photoconductive layer is formed in advance in a predetermined thickness on a discrete substrate, compared with the case where the photoconductive layer is directly formed (deposited) on the active matrix substrate by the vacuum vapor deposition method or coating method, the material, method, and condition of forming the photoconductive layer can be suitably selected from a wider selection (wider selection is available). That is, because the photoconductive layer is not directly formed on the active matrix substrate, the material, method, and condition of forming the photoconductive layer can be suitably selected from a wide selection irrespective of the heat resistant temperature of the active matrix substrate. Therefore, the photoconductive layer can employ various materials (photoconductive materials) for which, for example, the vacuum vapor deposition method was difficult to apply, and productivity of the photoconductive layer can be improved. Further, it is possible to easily form a photoconductive layer having superior uniformity in thickness and composition and having a (wide) thickness of, for example, 700 xcexcm to 3000 xcexcm. Further, since the photoconductive layer is not directly formed on the active matrix substrate, the photoconductive layer can also be formed easily on the active matrix substrate having a large area. As a result, it is possible to productively (efficiently) and inexpensively (easily) provide a two-dimensional image detector having superior uniformity in thickness and composition of the photoconductive layer with respect to the entire active matrix substrate.
A second object of the present invention is to provide a method of productively (efficiently) and inexpensively (easily) manufacturing a two-dimensional image detector having superior uniformity in thickness and composition of the photoconductive layer with respect to the entire substrate.
In order to achieve the second object, the fabrication method of the two-dimensional image detector in accordance with the present invention includes the step of forming the photoconductive layer on a discrete substrate and then transferring the photoconductive layer onto the active matrix substrate having a plurality of pixel electrodes.
With this method, it is possible to provide a fabrication method of the two-dimensional image detector having the foregoing effects.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.